Sensitizing silver halide emulsion layers

ABSTRACT

THIS INVENTION RELATES TO METHODS OF SENSITIZING SILVER HALIDE LAYERS, PHOTOGRAPHIC ELEMENTS COMPRISING SENSITIZERS WHICH CAN BE ACTIVATED AND TO PROCESSOS OF FORMING POSITIVE IMAGES. IN ONE ASPECT, A POSITIVE IMAGE IS FORMED IN A PROCESS COMPRISING (1) IMAGEWISE EXPOSING A SILVER HALIDE LAYER COMPRISING AN INTERNAL IMAGE EMULSION, (2) SURFACE-SENSITIZING THE INTERNAL IMAGE EMULSION, (3) FLASH EXPOSURE OF THE LAYER AND (4) DEVELOPING THE SILVER HALIDE LAYER.

United States Patent 01 Patented June 22, 1971 hce 3,586,505 SENSITIZING SILVER HALIDE EMULSION LAYERS Michael Ridgway, London, England, assignor to Eastman Kodak Company, Rochester, N.Y.

No Drawing. Filed Dec. 14, 1966, Ser. No. 601,534 Claims priority, application Great Britain, Dec. 16, 1965, 53,382/ 65 Int. Cl. G030 1/28, 5/24 US. Cl. 9664 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to photographicmaterials, their preparation and use. One aspect of this invention relates to methods of sensitizing silver halide emulsion layers and internal image silver halide emulsion layers. Another aspect of this invention relates to photographic elements which can be stored in a comparatively insensitive form and then sensitized just prior to or during use. Another aspect relates to direct positive photographic elements and their use.

Silver halide emulsions are usually sensitized before coating onto a support. Under certain adverse storage conditions such as radiation, however, such photographic elements become fogged and the element is of no use. It would be desirable to have a photographic element which could be stored in a comparatively insensitive form and then sensitized just prior to or during use.

Internal image silver halide emulsions are known to be useful in direct positive or reversal applications. The speed of such emulsions is very slow, however, and it would be desirable to find a way to increase the speed. The instant invention provides a solution to the above problems.

Accordingly, it is an object of this invention to provide a method for sensitizing a substantially unsensitized silver halide emulsion layer. Another object is to provide a method for sensitizing an internal image unsensitized silver halide emulsion layer. Another object is to provide unsensitized silver halide emulsions containing compounds which will sensitize the emulsions after an appropriate treatment. Still another object is to provide direct positive photographic elements having an internal image emulsion which has been sensitized according to the invention. Another object is to provide methods for using the direct positive elements of the invention. Yet still'another object is to provide unsensitized photographic elements which can be stored in a comparatively insensitive form and then sensitized just prior to or during use. Other objects will become apparent hereinafter.

According to the invention, a silver halide emulsion is prepared but not sensitized. Instead a selected sulfur sensitizing compound is merely added to the emulsion under conditions so that substantially no sensitization takes place, and the emulsion is thne coated onto a support. The photographic element can then be stored in this form and adverse storage conditions, e.g., radiation, will not cause fogging. When it is desired to sensitize the photographic element, all that need be done is contact the element with an alkaline medium such as ammonia vapors or alkaline solutions. Any alkaline solution can be used which is effective for the intended purpose. For example a 5% (by weight) sodium carbonate solution can :be used and it will cause sensitization in about ten seconds. The time of contacting the photographic element with the alkaline solution will, of course, depend upon the particular alkaline solution used. Between about ten seconds and five minutes has been found to be effective for most alkalnie solutions.

The sulfur compounds which can be added -to the silver halide emulsions of the invention are generally labile sulfur compounds such as the thioureas including s-diphenylthiourea, phenylthiourea, thiourea, allylthiourea, sdiethylthiourea, etc. which thioureas are preferred. Certain noble metal compounds, such as certain gold compounds, may also be employed with the sulfur compound as an auxiliary sensitizer. The concentrations of the sulfur compound can be any concentration which is effective for the purpose intended. Generally, a concentration of from about 1.0 mg. to about 50 mg. per mole of silver has been found to be elfective. The noble metal compound can also be used in any concentration effective for the intended purpose. For example, concentrations of from about 0.1 gram to about 10.0 grams of noble metal compound per sliver mole were found to be quite satisfactory.

In preparing an emulsion according to the instant invention, the sulfur compound is added in such a manner so that little of it reacts with the silver halide grains. A preferred way is to dissolve the compound in an aqueous gelatin solvent, set the solution obtained and then add it to the silver halide emulsion. It is also desirable to coat the emulsion as soon as it is prepared before any sensitization has taken place.

Normally, an ordinary coated emulsion layer is very difficult to sensitize since diffusion through the layers causes concentration gradients and hence uneven reaction. However, these problems are obviated by my invention since the sulfur compound is evenly distributed throughout the layer of the developing-out emulsion.

There are a number of adverse storage conditions which cause fogging of photographic elements. A particularly adverse condition is radiation. By use of the instant invention, the sensitization of the element just prior to use obviates this difficulty. Photographic film; used in space exploration may use this invention to obviate fogging caused by space travel, e.g. passing through the Van Allen radiation belt.

While the instant invention provides useful results with any silver halide emulsion, especially good results are obtainable with an internal image emulsion. An internal image emulsion is defined in US. Pat. 3,178,282 as one which, when measured according to normal photographic techniques by coating a test portion of the emulsion on a transparent support, exposing to a light intensity scale having a fixed time between 0.01 and 1 second, bleaching 5 minutes in a 0.3% potassium ferricyanide solution at F. and developing for about 5 minutes at 65 F. in Developer B, has a sensitivity, measured at a density of 0.1 above fog, appreciably greater (e.g., at least 1.4 log E greater) than the sensitivity of an identical test portion which has been exposed in the same way and developed for 6 minutes at 68 F. in Developer A. Several suitable emulsions of this type are described in Davey and Knott US. Pat. 2,592,250, issued Apr. 8, 1952. Silver chloride, silver bromide and silver chlorobromide emulsions of the internal latent image type can be used.

The following developing solutions were referred to hereinabove:

Grams N-methyl-p-aminophenol sulfate 0.31 Sodium sulfite, desiccated 39.6 Hydroquinone 6 Sodium carbonate, desiccated 18.7 Potassium bromide 0.86 Citric acid 0.68 Potassium metabisulfite 1.5 Water to make 1 liter.

N-methyl-p-arninophenol sulfate 2.0 Sodium sulfite, desiccated 90 Hydroquinone 8.0 Sodium carbonate, monohydrate a- 52.5 Potassium bromide 5 Sodium thiosulfate Water to make 1 liter.

Internal image emulsions can be used to produce direct positive or reversal photographic elements. For example, the process described in British Pat. 581,773 can be employed by imagewise exposing an internal image emulsion, flashing and then developing in a surface image developer. If desired, the emulsion may be contacted with the developer before flashing. The silver halide grains containing an internal latent image are apparently desensitized so that a uniform second exposure gives a positive latent image of the first exposure. The instant invention may be employed in such a process before the flashing and developing steps to produce an emulsion with a higher reversal speed. In addition, the maximum density and minimum density are greatly improved.

The instant invention may also be employed to produce a direct positive by use of an organic photo-reaction. An unsensitized emulsion is coated in the presence of a thiourea compound and a dye under conditions in which the thiourea does not react with the silver halide. It is then exposed imagewise. The thiourea is effectively removed by a photo-reaction which is catalyzed by certain dyes, including members of the triphenylmethane, cyanine 7 and azo dye classes, leaving an imagewise distribution of the thiourea. The emulsion is then either sensitized or fogged (depending on the amount of thiourea present) in the unexposed areas by bathing in a sodium carbonate solution. If necessary, a low level flash exposure is also given. Normal development in a surface image developer then gives a direct positive. Oxidizing agents such as potassium persulfate, hydrogen peroxide or benzoyl peroxide may also be incorporated into the emulsion to increase contrast and maximum density, and reduce minimum density.

The silver halide emulsions of the instant invention may also contain conventional addenda such as speed increasing addenda, e.g., quaternary ammonium salts, polyethylene glycols, thioethers, etc., gelatin plasticizers, coating aids, anti-foggants and hardeners such as aldehyde hardeners, e.g., formaldehyde, mucochloric acid, glutaraldehyde bis(sodium bisulfite), maleic dialdehyde, aziridines, clio-xane derivatives and polysaccarides. These emulsions may be useful in X-ray and other non-optically sensitized emulsions, and may also be used in orthochromatic, panchromatic and infrared sensitive emulsions. The addenda may be added to the emulsion before or after sensitizing dyes, if any, are used. Sensitizing dyes useful in sensitizing such emulsions are described, for example,

in US. Pats. 2,526,632 and 2,503,776. Spectral sensitizers which can be used are the cyanines, merocyanines, complex (trinuclear) cyanines, complex (trinuclear) merocyanines, styryls and hemicyanines. Developing agents can also be incorporated into the silver halide emulsions if desired or be contained in a separate underlayer. Various silver salts may be used as the sensitive salt such as silver bromide, silver iodide, silver chloride, or mixed silver halides such as silver chlorobromide or silver bromoiodide. The silver halides used can be those which form latent images predominantly on the surface of the silver halide grains or those which form latent images inside the silver halide crystals as exemplified by Davey and Knott US. Pat. 2,592,250. 1

The silver halide emulsions of the instant invention can contain any of the hydrophilic water-permeable binding materials suitable for this purpose. Suitable materials include gelatin, colloidal albumin, polyvinyl compounds, cellulose derivatives, acrylamide polymers, etc. Mixtures of these binding agents can also be used. The binding agents for the emulsion layer of the photographic element can also contain dispersed polymerized vinyl compounds. Such compounds are disclosed, for example, in US. Pats. 3,142,568; 3,193,386; 3,062,674 and 3,220,844, and include the water-insoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates and the like.

The silver halide emulsions of the instant invention may be coated on a wide variety of supports. Typical supports are cellulose nitrate film, cellulose ester film, polyvinyl acetal film, polystyrene film, poly(ethylene terephthalate) film and related films or resinous materials as well as glass, paper, metal and the like. Supports such as paper which are coated with a-olefins polymers, particularly polymers of a-olefins containing two or more carbon atoms, as exemplified by polyethylene, polypropylene, ethylene-butene copolymers and the like can also be employed.

The following examples will illustrate the invention, but are not to be construed to limit it in any way.

EXAMPLE 1 A silver chlorobromide developing-out emulsion containing 2% gelatin was prepared and was added to an equal volume of 20% gelatin solution giving an emulsion of 23 grams equivalent silver nitrate per kilogram of emulsion. The emulsion was melted and brought to 40 C. when the pH was approximately 7.0. Successive additions of a set 10% solution of an inert gelatin containing 0.1% thiourea were added at four minute intervals, each addition representing approximately 5 mg. of thiourea per mole of silver halide. A sample of emulsion was removed and coated three minutes after each addition of sensitizer. Portions of the coatings were exposed to tungsten light for one minute and then processed; they had very low photographic sensitivity. Further portions were bathed in a 5% carbonate solution (ph 11.2) for 5 minutes before exposure: speed and gamma increases at optimum thiourea concentration close to those obtained by normal liquid emulsion digestion techniques were obtained. The optimum concentration of thiourea was about 15 mg. er mole of silver halide. A similar speed increase was obtained irrespective of whether the coating was wet with carbonate, wet after rinsing or washing to remove the carbonate, or dry after removal of carbonate.

EXAMPLE 2 An emulsion of the type in British Pat. 581,773 in which the internal sensitivity was much higher than the surface sensitivity was melted and brought to 40 C., the pH being 6.5 and pAg being 8.0. Thiourea was added at various concentrations as described above. Optimum sensitivity was reached with a thiourea concentration of about 10 mg. per mole of silver halide. The emulsion was coated on film base and a sample of the coating was imagewise exposed. The exposed sample was sensitized by bathing for ten seconds in sodium carbonate solution (this effected a very large increase in speed), and then exposed uniformly to tungsten light. Development in a surface image developer, (e.g., Formula 1 of British Pat. 581,773) at room temperature for five minutes produced a direct positive image of good quality.

A comparison test was carried out using a material prepared from a portion of the emulsion to which no thiourea had been added. After the imagewise exposure, the material was treated with a carbonate solution. The quality of that image was markedly inferior to that of the image obtained with the material of the present invention, the maximum density being lower and the minimum density being higher than that of the present invention. Furthermore, the sensitivity of the sensitized material of this invention was much greater than that of the comparison material after treatment with the carbonate solution.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected without departing from the spirit and scope of the invention as described herein above and as defined in the appended claims.

I claim:

1. A method for obtaining a positive image comprising:

(a) imagewise-exposing a photographic element comprising at least one layer containing an internalimage silver halide emulsion wherein the internal sensitivity is higher than the surface sensitivity;

(b) contacting the silver halide grains of said internalimage emulsion with a silver halide chemical sensitizing compound which contains labile sulfur atoms under conditions whereby the chemical sensitizing compound containing labile sulfur atoms will react with said grains;

(0) forming a latent image on the surface of the unexposed silver halide grains; and

(d) developing said element in a surface image silver halide developing composition.

2. A method according to claim 1 wherein said internal-image emulsion is unsensitized on the surface vof the silver halide grains prior to said imagewise exposure.

3. A method according to claim 1 wherein said compound containing a labile sulfur atom is a thiourea compound.

4. A method according to claim 1 wherein said compound containing a labile sulfur atom is present in the silver halide element before imagewise exposure under conditions wherein it does not react with the silver halide grains and said compound is activated after imagewise exposure by contacting it with an alkaline medium.

5. A method according to claim 1 wherein said photographic element is given a uniform light exposure after the imagewise-exposed silver halide emulsion has been reacted with said compound containing labile sulfur atoms and prior to the completion of development in a surface image developer.

6. A method according to claim 1 wherein said photographic element comprises from about 1.0 mg. to about 50 mg., per mole of silver, of said compound containing labile sulfur atoms and conditions are maintained in said element whereby substantial reaction of said compound containing labile sulfur atoms with the silver halide grains will not occur before the imagewise-exposure step.

7. A method for obtaining a positive image in a photographic element comprising:

(a) imagewise-exposing a photographic element comprising at least one layer containing an internalimage silver halide emulsion, a test layer of which coated on a transparent support and the layer exposed on a light-intensity scale having a fixed time between 0.01 and 1 second, bleaching for 5 minutes in a 0.3% potassium ferricyanide solution at 65 F. and developing for about 5 minutes at 65 F. in the following developer:

G. N-methyl-p-aminophenol sulfate 2.0 sodium sulfite, desiccated 90.0 hydroquinone 8.0 sodium carbonate, monohydrate 52.5 potassium bromide 5.0 sodium thiosulfate 10.0

water to make 1 liter.

has a sensitivity, measured at a density of 0.1 above fog, at least 1.4 log E greater than the sensitivity of an identical test portion which has been exposed in the same way and developed for 6 minutes at 68 F. in the following developer:

G. N-methyl-p-aminophenol sulfate 0.31 sodium sulfite, desiccated 39.6 hydroquinone 6.0 sodium carbonate, desiccated 18.7 citric acid 0.68 potassium metabisulfite 1.5

water to make 1 liter.

(b) sensitizing the surface of the silver halide grains in said internal-image emulsion by contacting it with a sulfur sensitizer, which comprises a labile sulfur atom, under conditions wherein said sulfur sensitizer will react with the silver halide grains to form lightsensitivity sites on the surface of at least some of the silver halide grains;

(c) exposing said photographic element uniformly to light; and

(d) developing said silver halide element in a silver halide, surface image developing composition.

8. A process according to claim 7 wherein said sulfur sensitizer is thiourea.

9. A method according to claim 7 wherein said sensitization is carried out under conditions of an alkaline medium.

References Cited UNITED STATES PATENTS 3,178,282 4/1965 Luckey et al. 96'64 3,189,449 6/1965 Yost 96-28 3,338,712 8/1967 Thurston 96107X 3,367,778 2/1968 Berriman 96-64 OTHER REFERENCES Some Chemical Factors in Emulsion Sensitivity in Photographic Sensitivity, published by Butterworths Scientific 'Publication, London, 1951, pp. 112-113, TRS C6 1950 c. 2.

NORMAN G. TORCHIN, Primary Examiner R. E. FICHTER, Assistant Examiner US. Cl. X-R. 96107 @3 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,586,505 Dated June 22, 1971 Inventor(s) Michael Ri dgway It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

F "1 In column 2, line 16 (page 2, line 23 of the application) "alkalnie" should read --alkaline---.

In column 2, line 37 (page 3, line 21 of the application) "solvent" should read --solution.

In column 6, line 29 of Claim 7 under- "sodium carbonate, desiccated 18.7" insert ---potassium bromide Signed and sealed this 7th day of March 1972.

(SEAL) Attest':

EIEIZARBMJLRTCHERJR. ROBERT GOTTSCHALK es lng Officer Commissioner of Patents 

